This article has been accepted for inclusion in a future issue of this journal.

Content is final as presented, with the exception of pagination.

IEEE COMMUNICATIONS SURVEYS & TUTORIALS, ACCEPTED FOR PUBLICATION

The Evolution of Cellular Backhaul Technologies:

Current Issues and Future TrendsOrawan Tipmongkolsilp, Said Zaghloul and Admela Jukan

AbstractThe rapid increase of the number of mobile subscribers as well as the deployment of 3G technologies are puttingstrain on mobile backhaul operational expenditures (OPEX)which amount to 20-40% of total mobile operators OPEX dueto their reliance on T1/E1 copper lines. For these reasons, thecurrent backhaul systems, a term commonly used to describeconnectivity between base stations and radio controllers, areincreasingly integrating more cost-effective, packet switched technologies, especially Ethernet/Internet technologies. In addition,Wi-Fi and WiMAX are emerging as promising backhaul solutionsand initial findings have demonstrated their feasibility. However,the notion of network migration unavoidably raises new technicalchallenges relevant to aspects of TDM and packet network timingsynchronization, QoS, and packet efficiency. This survey aims toprovide a comprehensive study of state-of-the-art circuit switchedand emerging packet switched backhaul technologies based onresearch articles and standard documents. For packet switchedbackhaul, we focus on the practically important Pseudowire approaches which are used to transport TDM services over packetswitched networks. We also discuss the features and researchfindings on the use of Wi-Fi and WiMAX technologies whichillustrate their potential for rapid and cost-efficient backhauldeployment. Finally, we highlight some open issues relevant totiming synchronization in wireless mesh backhaul and femtocellsdeployments, which offer a rich ground for further research.Index TermsBackhaul networks, GSM, Mobile networks,circuit switching, packet switching, time synchronization.

I. I NTRODUCTIONOBILE backhaul is a term commonly used to describeconnectivity between base stations and radio controllersin cellular systems over a variety of transport media. Asillustrated in Figure 1, todays backhaul relies mostly on threephysical mediums: copper, optical fiber and microwave radiolinks. In the US, for example, leased T1/E1 copper constitutesapproximately 90% of backhaul implementations followed bymicrowave links (about 6 %) and optical fibers (about 4 %)(1). Optical fibers may be deployed in dense urban and suburban locations, which are considered high traffic areas. On theother hand, microwave radio and satellite links are utilized inlocations where wired backhauls are difficult to deploy. LeasedT1/E1 copper lines dominate the backhaul solutions, as theyprovide suitable support for voice traffic, with deterministicQoS, low latency and low delay variations (jitter). In addition,timing and synchronization is inherently available from T1/E1lines, which is a necessary requirement in cellular systems.Up to now, one to two leased T1/E1 copper lines have been

considered sufficient per cell site to handle 2G traffic including

voice and short message service.Recently, however, the required backhaul capacity has significantly increased due to the increasing number of mobilesubscribers and the availability of mobile high-speed dataservices. The increasing number of mobile subscribers haveresulted in a significant growth in the number of deployedbase station sites and associated T1/E1 connections. In theUS, for example, the number of base stations went up from30,045 in 1996 to 213,299 in 2007 (1). On the other hand, theimplementation of GPRS and EDGE requires four times thenumber of leased T1/E1 copper lines, compared to five yearsago and is expected to be as much as eight to sixteen timeswhen HSPA and LTE 4G technologies are fully deployed. As aconsequence, leased T1/E1 copper, which price increases linearly with capacity, is not a cost efficient choice for backhaul.Today, the backhaul expenditure remains one of the greatestconcerns for mobile operators. Due to their significance andimpact, some have recently referred to it as the "telecomglobal warming" (2). This problem caused some equipmentproviders to resort to workforce reductions in the accessnetwork business and to shift resources to the mobile backhaularea (3), and motivated many mobile operators to migratetowards cost effective packet-based backhaul solutions.In this survey, we offer a comprehensive study of the ongoing migration from legacy to emerging backhaul networktechnologies, which is to the best of our knowledge, the firstsurvey in this emerging area. Our survey presents a thoroughexamination of numerous cellular and Internet standards aswell as relevant publications from academia and industry.Based on the survey study, we unveil the potential andthe challenges of evolving backhaul solutions using packetswitched networks and inexpensive wireless technologies,such as Wi-Fi. We show that the migration to new technologies raises new technical challenges relevant to QoS, packetefficiency, and timing synchronization. From Pseudowire towireless mesh, we identify attractive features of the emergingsolutions, with respect to their low cost and availability. Especially interesting are the recent implementations of wirelessmesh backhaul solutions using vendors proprietary protocolsin commercial sites (4). Also the current "all-IP" trends in 4Gnetworks and femtocells carry interesting research challengesfor wireless backhaul.This survey is organized as follows. In Section II, wepresent traditional and emerging backhaul technologies. Westart by explaining advantages and shortcomings of traditional backhaul technologies (e.g., Leased T1/E1 copper, optical fiber, microwave and satellite). We then discuss Pseu-

IEEE COMMUNICATIONS SURVEYS & TUTORIALS, ACCEPTED FOR PUBLICATION

dowire solutions based on emerging backhaul technologies

for transporting Time Division Multiplexing (TDM) overpacket switched networks along with the associated technicalchallenges. In Section III, we study prospective wireless technologies for backhaul solutions based on Wi-Fi and WorldwideInteroperability for Microwave Access (WiMAX). In SectionIV, we survey timing and synchronization in current andemerging backhaul solutions. Section V summarizes the mainfindings from this survey by comparing of all presentedbackhaul technologies, and discussing future trends and openissues. Section VI concludes the paper.II. F ROM T RADITIONAL TO E MERGING BACKHAULT ECHNOLOGIESIn this section, we start by addressing wired backhaulnetworks based on copper cables and optical fibers, give anoverview of wireless backhaul technologies such as microwaveand satellite, and finally conclude the section by an overviewof the Pseudowire technology.A. Copper and Optical Fiber Backhaul NetworksCopper cables are the traditional backhaul medium betweenBase Transceiver Stations (BTSs) and Base Station Controller(BSC). TDM techniques using the so-called PlesiochronousDigital Hierarchy (PDH) are prevalent techniques which allowmultiplexing multiple voice channels from base stations andtransporting them to the BSC in different time slots. Inthis regard, there are two standard plesiochronous hierarchieswhich are very similar in their operation but primarily differin the delivered bit rates: the T-carriers (T1,T2,...,T4) and Ecarriers (E1,...,E5). For instance, T1 links operate on 1.544

Mbit/s while E1 connections operate on 2.048 Mbit/s. The

T-carriers are primarily used in North America and Japanwhile E-carriers are used in Europe and the rest of the world.T1/E1 connections can be deployed as point-to-point systemsor over PDH multiplexing systems. The T1 frame consistsof 24 time slots of DS0; each can support a 64 kbit/s PCMvoice call. Typically one DS0 is dedicated for signaling;therefore, without any voice compression, a T1 line can carry23 voice calls. For E1, there are 32 time slots of DS0 where30 time slots are used for voice communications and theother two are used for frame synchronization and signaling1.For better bandwidth utilization, voice compression techniquessuch as G.729 and EVRC are introduced to compress 64 kbit/sPCM encoded voice streams, leading to a throughput gain offour folds (i.e., one DS0 can support 4 PCM streams usingcompression). As a result, one T1 can support 23x4 = 92 voicecalls and one E1 can support 30x4 = 120 voice calls.Optical fibers. In many cases leased T1/E1 copper linesfrom multiple sites are merged at a multiplexer which multiplexes lower rate T1/E1 connections into higher rate opticalfiber connections such as STM-1 (155.52 Mbit/s), STM-4(622 Mbit/s) and STM-16 (2.4 Gbit/s) as shown in Figure2(a). The STM standards are used as Synchronous OpticalNetworking (SONET) in North America and as SynchronousDigital Hierarchy (SDH) in Europe and the rest of the world2 .As also shown in Figure 2, TDM backhaul can distributetiming information throughout the network. In cellular systems1 Time slot 0th is dedicated for frame synchronization and time slot 16this dedicated for signaling.2 SDH and SONET are standardized multiplexing protocols for transferringdigital streams over point-to-point optical fibers and radio links, dependingon the specific operator choices.

This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.O. TIPMONGKOLSILP et al.: THE EVOLUTION OF CELLULAR BACKHAUL TECHNOLOGIES: CURRENT ISSUES AND FUTURE TRENDS

(a) TDM Backhaul Network Using Aggregation of Point-to-Point Leased

Lines

(b) TDM Backhaul Network Using SDH/SONET over Optical Ring

such as GSM, the primary reference clock (PRC) (i.e., the

master clock signal) maybe hosted by the mobile switchingcenter (MSC) or at the base station controller sites3 andthe slave clocks (a.k.a, secondary clocks) at the base stationsites are traceable to the primary clock reference. The issuethat arises now is that as we move towards packet switchednetworks using Pseudowire technologies, timing informationis lost. This is a serious issue and if not mitigated, base stationsmay not be able to control their radio frequencies properly andhandoffs may result in dropped calls. Network synchronizationis very important for proper handover process in cellularnetworks. More details of timing and synchronization aspectwill be discussed in Section IV-B.SDH/SONET over optical fibers can also be implementedin ring topologies, as shown in Figure 2(b). The add/dropmultiplexer is an important element of an optical fiber networkwhich combines or multiplexes several lower-bandwidth datastreams into a single beam of light. In addition, it can addone or more lower-bandwidth signals to an existing highbandwidth data stream, while at the same time, extract ordrop other low bandwidth signals by removing them from thestream and redirecting them to other network paths. The useof SDH/SONET fiber rings can only be justified at the cellularsites when certain cost conditions are satisfied, as discussed in(5), where it was suggested to replace T1 connections at thecellular sites based on a multi-parameter backhaul cost model.The parameters include the distance between the cell site andthe add/drop multiplexer, the number of T1 connections persite, and the number of cell sites to be connected via the fiberring. The results indicate that a ring should at least serve 4 cellsites for cost efficiency and that optical backhaul can achievesignificant cost savings (27% or more) for cell sites with 4 ormore T1 connections.

3 In some case (e.g., GPS primary clocks), the primary clock reference isdistributed and is directly available at the base station sites.

B. Microwave and Satellite Wireless Backhaul

Microwave radio links are an alternative choice for wiredbackhaul links especially in geographically challenging areas where wired connections are not available. Microwavetransmission can be carried out in various frequency bandsincluding licensed (6 GHz to 38 GHz) and unlicensed (2.4GHz and 5.8 GHz) bands. Using unlicensed bands can reduceCapital Expense (CAPEX) but raises radio interference issues.The used frequency spectrum affects bandwidth capacity anddistance coverage; the higher the frequency, the greater thebandwidth capacity and the shorter the coverage range4. In allcases, the presence of Line of Sight (LOS) between cell citesand aggregation points (e.g., at a SONET ring) is requiredand hence microwave is limited to short distance transmissionwhen used in metropolitan environments. However, in ruralenvironments, when a LOS is present, microwave transmissioncan be quickly installed to cover long distances. Compared toT1/E1 copper links, implementing microwave links results inhigher CAPEX due to equipment costs and spectrum licensingfees, however they are likely to incur less OPEX over time.Microwave can be implemented in the Point-to-Point (PTP),Point-to-Multipoint (PMP), or proprietary multihop configurations for better coverage. Whereas the PTP system requires aradio and antenna at the end of every wireless link, in PMP,one radio and antenna at an aggregation point are sufficient toserve a number of cell sites. The digital transmission techniqueover microwave links can be based on PDH (i.e., one or moreT1/E1), SDH/SONET or Ethernet (Gigabit Ethernet protocol).Figure 3 shows the PTP microwave backhaul network withincreased distance coverage as a result of implementing amultihop architecture. Research results in (7) suggest thatusing PTP microwave links to backhaul traffic from cell sites4 Some products (6) can provide wireless Gigabit Ethernet Point-to-Point(PTP) communications with throughput up to 1.25 Gbit/s in the 71-76 GHzand 81-86 GHz bands, and rates up to 170 Mbit/s in the 17.7-19.7 GHzmicrowave band.

TABLE I: Pseudowire Standards

Fig. 3: Point-to-Point Microwave Backhaul (BTS: Base

onto leased T3 copper links can result in significant savings

compared to using leased T1 copper backhaul. In addition,deploying a PMP topology in microwave backhaul networkcan only be cost efficient if at least 5 cells are served by eachPMP system.Satellite backhaul offers a solution for locations where noother backhaul technologies are feasible. The transmissionover satellite links for cellular backhaul is mostly based onT1/E1 techniques. Advantages of satellite links are their shortinstallation times and flexible coverage while their majordrawbacks are their high cost and long propagation delay. Forinstance, based on (8), typical propagation delays for satellitelinks are around 270 ms plus processing delay, which arehigher than the acceptable end-to-end delay for voice servicesof 250 ms. In addition, the cost for the transponder bandwidthof 768 kbit/s (about a half of T1) on a full time basis is $3,000- $6,000 per month and for an equivalent of full T1 bandwidth(1.544 Mbit/s) is $5,000 - $12,000 per month (data as of2002). To alleviate such high expenses, usage based billingmechanisms was proposed to help reducing the incurred costs.This is achieved by using Demand Assigned Multiple Access(DAMA) techniques which dynamically allocate bandwidthresources based on actual requests from the users.The feasibility of satellite cellular backhaul was demonstrated in challenging sites such as islands and remote locations where no wired solutions are available. For instance, in(9), the authors presented a case study of using satellite ascellular backhaul in the Federated States of Micronesia in thePacific Ocean. They showed that the satellite backhaul is muchmore cost effective than deploying costly submarine cables.Another example is the satellite backhaul trials which tookplace in rural areas in France (10). The results of the trialsnot only demonstrated the feasibility of the satellite link butalso that the traffic in forward and reverse links is symmetricalwhich offers useful input to satellite bandwidth management.C. The Pseudowire FrameworkTraditional backhaul technologies described so far majorallyrely on circuit switched technologies and hence can not bedirectly connected to packet networks. As the next generation

RFC 5087

Description Describes an architecture for Pseudowire Discusses the emulation of services e.g. TDM,ATM over packet switched networks Describes the design of a Pseudowire ControlWord for use over MPLS to distinguish Pseudowirepayload from a regular IP payload Specifies the encapsulation of Ethernet payloadto be carried over MPLS Specifies the procedure for using a Pseudowireto provide a point-to-point Ethernet services Specifies the encapsulation of ATM cells to becarried over MPLS Describes a transparent cell transport service forencapsulating ATM cells to be carried over packetswitched networks Provides encapsulation formats for emulatingSDH/SONET services over MPLS Describes a method for encapsulating TDMbit streams to be carried over packet switchednetworks Provides encapsulation details of TDM payloadfor specific packet switched networks e.g. MPLS,Ethernet.

LTE systems and alike are expected to natively use packetbased backhauls, the Pseudowire framework was introducedas a backhaul technology to transport traditional services, e.g.TDM over packet switched networks, e.g. Ethernet, IP orMPLS (11). Relevant to our discussion, the mechanism oftransporting TDM traffic over a packet switched network isreferred to as circuit emulation, a.k.a, TDM Pseudowire.It is widely accepted that Pseudowire techniques not onlyoffer better network integration over unified packet switchedcores, but they also offer significantly lower cost per megabit,with a cost/megabit ratio for a T1/E1 circuit to Ethernet ofapproximately 6 to 1 (12)). In fact, several standards werepublished by the IETF to date, such as RFC 5086 and RFC5087 (13; 14) which address circuit emulation for GSM backhaul; RFC 4717 and RFC 4816 (15; 16) address the transportof ATM services used in UMTS backhaul over packet switchednetworks. Many Pseudowire standards were recently publishedby the IETF Pseudowire Emulation Edge to Edge (PWE3)working group which address architecture, native services(e.g., TDM), framing protocols, types of packet switchednetworks (Ethernet, IP, MPLS), and operational aspects. Someof these standards are listed in Table I.Figure 4(a) illustrates how the native service payload isfirst processed by the encapsulation layer, the Pseudowire(PW) de-multiplexer layer and the Packet Switched Network(PSN) convergence layer. The resulting Pseudowire data isthen encapsulated in the data portion of the packets traversingthe packet switched network. In the context of backhaul, thePseudowire payload types can be a packet (e.g., ATM AAL5PDU), an ATM cell, a T1/E1 or a T3/E3 bit stream, or astructured bit stream (e.g., SDH/SONET). The encapsulationlayer provides any information that is needed by the edgedevices to forward packet from the packet switched networkboundary to the TDM network boundary and to reconstruct

This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.O. TIPMONGKOLSILP et al.: THE EVOLUTION OF CELLULAR BACKHAUL TECHNOLOGIES: CURRENT ISSUES AND FUTURE TRENDS

(a) Pseudowire Protocol Architecture,

(BSC: Base Station Controller, RNC: Radio Control Network)

(b) Pseudowire Payload Encapsulation (AAL: ATM Adaptation Layer)

the original TDM payload from the received packets (13).

The PW Demultiplexer layer allows delivering multiple Pseudowire connections over a single packet tunnel. The PSNconvergence layer allows Pseudowires to be independent ofthe packet switched network type while meeting the servicerequirements. Significant relevant functionality to backhaulnetworks is inherent to the encapsulation layer as describednext.The Encapsulation layer is composed of three sub-layers:Payload Convergence, Timing, and Sequencing. The maintask of the payload convergence sub-layer is to encapsulatethe payload. The timing sub-layer provides timing and synchronization within packet switched networks (see SectionIV-B for details). The sequencing sub-layer handles out-oforder packet arrival and packet loss issues. In the context ofbackhaul networks, the encapsulation layer can be used tohandle the GSM TDM based backhaul circuits as well as theUMTS ATM based backhaul connections. As shown in Figure4(b), the encapsulation layer decides the type of processingdepending on the payloads type(e.g., whether it is a T1/E1bit stream or an ATM cell). For a GSM T1/E1 bit streampayload, the TDM frames are converted to packets along withsome added headers, including a PSN header, a Real-timeTransport Protocol (RTP) header, and a TDMoIP control wordas follows,The PSN header may be IPv4/IPv6+UDP header orMPLS label stack. RTP is normally used to transport timing informationacross the packet switched network by providing a sequence number and a timestamp (13); but it is practicallyomitted due to two reasons. First, the TDM sourceproduces a constant bit rate based on its local clock; andsecond, the size of the RTP header is relatively large (12bytes) (14). TDMoIP control word consists of sequence number,payload length, and flags. The sequence number is usedfor frame re-ordering and packet loss detection while theflags are used to indicate error conditions (see (13)).The encapsulation of TDM bit streams is not a mere

packetization process since lost packets can cause service

interruption and due to the packetization inefficiency incurredwhen encapsulating fractional T1/E1 frames (14). Usually,multiple T1/E1 frames are grouped together into one bigframe before encapsulation and only the timeslots in usewithin each T1/E1 frame are sent (14). The AAL1 and AAL2encapsulation techniques are used to handle the bit stream.AAL1 is suitable for Constant Bit Rate (CBR) applicationswhere timeslots are statically allocated, while AAL2 is suitablefor Variable Bit Rate (VBR) where timeslots are dynamicallyassigned (14). According to RFC 5087, the specific choiceof AAL techniques rather than other encapsulation methodsis due to three primary reasons. First, AAL mechanisms aregeneral solutions for transporting constant or variable-ratereal-time streams over packet switched networks. Second,AAL mechanisms are already deployed within and at theedge of the public telephony system. Third, the use of AALtechnologies simplifies interworking with existing AAL1 andAAL2 based networks (14). These mature technologies areproven to reliably transfer voice-grade channels, data, andtelephony signaling.Similar to the encapsulation process of TDM payload, theATM payload (see Figure 4(b)) is encapsulated with addedheaders including a PSN header, a Pseudowire header, andan ATM control word (15). The PSN header depends onthe used tunneling technology (e.g. IPv4/IPv6 or MPLS).The Pseudowire header identifies a particular ATM servicewithin the PSN tunnel (15). The ATM control word containsa length, a sequence number, and control bits needed to carrythe service. Since the traffic flows between the Radio NetworkController (RNC) and Node B are carried over an Iub interfacewhich includes signaling and data, signaling is handled usingAAL5 while data is handled using AAL2 encapsulation rules.Although migrating to packet switched networks for cellularbackhaul offers significant advantages, it poses technical issuesrelevant to timing synchronization, quality of service (QoS),and packet efficiency. Due to the breadth of the details relevantto timing synchronization, we address them separately inSection IV. QoS issues arise as packet switched networks

This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.6

IEEE COMMUNICATIONS SURVEYS & TUTORIALS, ACCEPTED FOR PUBLICATION

are predominantly best-effort and connection-less, which turn

them difficult to offer QoS guarantees. The survey of researchresults on QoS in IP-based networks can be found in (17).With the inception of MPLS RSVP-TE as a connectionoriented switching paradigm, it became possible to offerend-to-end QoS with fast packet delivery in a way that iscomparable to ATM networks.Packet efficiency is another important issue that arises aslarge protocol overheads are added to the service payload inpacket switched networks. While packet efficiency is not anissue for data packets with large payload sizes, it is a seriousproblem for data packets with low payloads (e.g., voice) wherethe packet header size is comparable to the payload size.For example, to meet the stringent delay requirement (<100ms delay) using EVRC codec (9 kbit/s) in EV-DO cellularsystems, voice packets are sent every 20 ms. This results in avoice payload of 24 bytes and a UDP/IP header of 28 bytes(20 bytes IP + 8 bytes UDP) resulting in 53% of overhead.In order to mitigate this issue, header reduction techniqueswere proposed, e.g., in (18). In addition, IETF standardswidely addressed efficiency issues for VoIP traffic (e.g., RFC2507, RFC 2508, and RFC 3095 Robust Header Compression(ROHC)). For example, RFC 2508 reduces the 40 bytesRTP/UDP/IP header to 2 bytes while ROHC reduces it to 1byte. The RFC 2508 technique is sensitive to packet losseswhile ROHC is robust against packet losses and propagationdelay but does not handle out-of-order packet arrivals (19).Pseudowire, however, can handle out-of-order arrivals byeither dropping the out-of-order packets or by re-sequencingthe packets into the correct order when possible. For packetloss issues, Pseudowire has a frame loss detection feature thatenables the receiving end to recognize losses by tracking thesequence number of the received packets (20). Once a lossis detected, the Packet Loss Concealment (PLC) technique isapplied to replace the lost bits by either substituting the lostbits with zero bits or by replicating the previous byte stream.III. P ROSPECTIVE W IRELESS S OLUTIONS FOR BACKHAULWith the advent of Pseudowire technologies described inthe previous section, researchers are seeking innovative waysto utilize new broadband wireless solutions such as Wi-Fiand WiMAX as prospective technologies for cellular backhaulnetworks. In this section, we give a short overview of WiFi and WiMAX wireless technologies and discuss throughputperformance, distance coverage, packet efficiency, and QoS asrelevant to their applicability to backhaul systems. We leavethe discussion on timing and synchronization to Section IV-B.A. Wi-Fi Network TechnologyWi-Fi was originally designed for indoor usage based onthe IEEE 802.11 standards and operates in the 5 GHz and 2.4GHz unlicensed bands. Recent research advances in industryand academia demonstrated the feasibility of long distance WiFi connectivity up to 38 km (2123). As such, Wi-Fi can be anattractive low cost solution for backhaul which can substitutemicrowave links. As shown in Figure 5, Wi-Fi links can beused in combination with Pseudowire to backhaul traffic fromnearby cell sites to the radio network controller residing in

Fig. 5: Wi-Fi Backhaul Network

the core network. A base station may have a Wi-Fi interface

with built-in Pseudowire capabilities as in site "B" and candirectly backhaul its traffic back to the nearest aggregationaccess point or an additional unit as in site "A". Notice thatin our example, site "A" and site "B" use non-overlapping WiFi channels (e.g., channel 1 and channel 11) to interact withthe multi-radio aggregation access point to avoid collisions.Although Wi-Fi technology offers attractive cost benefits anddeployment flexibility for backhaul networks, it poses designissues relevant to the achieved throughput, distance coverage,packet overhead, and timing and synchronization.Relevant to throughput, several Wi-Fi router manufacturerssuch as (24), have added proprietary extensions to their 802.11products, to support up to 108 Mbit/s. In addition, the newIEEE 802.11n standard significantly improves the networkthroughput of up to 600 Mbit/s by using Multiple InputMultiple Output (MIMO) technologies, which use multipleantennas at both transmitter and receiver. A comprehensiveliterature survey of MIMO techniques can be found in (25).The Spatial Reuse TDMA (STDMA) technique (26; 27)was proposed in (21) to increase the capacity of Wi-Fi byscheduling links for various simultaneous transmissions. Theidea behind STDMA is that when wireless nodes are spreadout geographically, then the same time slot can be usedfor concurrent transmissions between nodes with sufficientspatial separation (27; 28). In addition, traditional frequencyplanning is required to enhance the network capacity, whereusing multiple non-overlapping channels allows simultaneouscommunications between nodes without interference (2931).Finally, frequency reuse techniques can be used such thatthe same Wi-Fi channels are used in sufficiently separatedcommunicating groups. Therefore the throughput achievedin Wi-Fi networks offers a potential solution to aggregatebackhaul connections from a number of base station sites backto the core network at a low cost.As Wi-Fi was originally designed for indoor usage, usingit for point-to-point long-distance links, which is relevantto backhaul, was an active area of experimental research inthe past few years. In (21; 22), the authors built a testbedin a rural setting and demonstrated that a 38 km long

This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.O. TIPMONGKOLSILP et al.: THE EVOLUTION OF CELLULAR BACKHAUL TECHNOLOGIES: CURRENT ISSUES AND FUTURE TRENDS

link (802.11b) is feasible using highly directional antennas

(23-dBi parabolic grid antenna placed on top of a 40 mtower). In addition, several industrial vendors (e.g. (23)), offer802.11 commercial products to provide long distance outdoorcoverage, albeit with proprietary modification of the 802.11MAC protocol. For example, in (23), long distance 802.11gis offered with throughput up to 22 Mbit/s over 10 km inOrthogonal Frequency Division Multiplexing (OFDM) mode,and approximately 40 km in Direct Sequence Spread Spectrum(DSSS) mode at lower rates.The usage of long distance Wi-Fi connections, however,raises several technical challenges in both the physical andMAC layers. Specifically, in the physical layer, 802.11b usesthe DSSS modulation techniques where transmission is spreadover a large bandwidth and is hence susceptible to multipathfading (21). In OFDM mode, on the other hand, multiplenarrow channels are used to transmit data, where each channelis modulated with a low data rate sub-carrier, and the sum fromall channels yields a high data rate. OFDM sub-carriers have along symbol duration; hence multi-path problems are avoidedas long as the delay spread is below the symbol duration.At the MAC layer, many parameter values are not suitable foroutdoor scenarios, e.g., acknowledgement timeout, ContentionWindow (CW) and round-trip time. This is because in 802.11MAC, Acknowledgement (ACK) packets must be sent toensure successful delivery. The default ACK timeout valueof 20 s is too short for long distance links (the propagationdelay over 15 km is 50 s) (21). Nevertheless, the achieveddistances of 10 km and 40 km carry potential to backhaulnearby sites at low cost as well as decently distant sites insuburban environments.Wi-Fi packet efficiency comes into play when using WiFi to backhaul voice traffic. To illustrate this issue, let ususe the same EVRC codec VoIP example, which offers VoIPpayload sizes of 24 bytes. We assume that ROHC header compression techniques are used to reduce RTP/UDP/IP headerssize from 40 bytes to 1 byte leading to VoIP packet sizeof 25 bytes. The Wi-Fi MAC frame format including framecontrol, duration ID, addresses, sequence control, data andFrame Check Sequence (FCS) fields adds 28 bytes to thepayload resulting in 53% MAC overhead. Notice that Wi-FiPHY layer with PLCP preamble and header leads to another 24bytes of overhead. To address this issue and enhance the efficiency, concatenation techniques are introduced in (32) whichencapsulate multiple voice payloads into one PHY frame withone MAC header. Therefore, concatenation techniques as wellas header compression techniques should be considered inbackhaul applications.To support the required QoS in a backhaul application, WiFi must be deployed beyond the standard configurations (e.g,.802.11b, 802.11g) which rely on the Distributed CoordinationFunction (DCF) MAC protocol for channel access5 . SinceDCF does not support QoS, the IEEE 802.11e amendmentwas introduced to support QoS in Wi-Fi networks. In thisregard, there are two methods for channel access: Hybridcoordination function Controlled Channel Access (HCCA) and5 Point Coordination Function (PCF) was proposed in infrastructure modefor centralized contention control, however, only a few Access Point (AP)shave enabled this feature in practice.

Enhanced Distributed Channel Access (EDCA). HCCA is a

centralized contention scheme and EDCA is a distributed priority based contention scheme. In the latter, each node assignshigh priority traffic (e.g., VoIP) shorter backoff times thanbest effort traffic. Both schemes have traffic differentiationfeatures. However, HCCA support is not mandatory in 802.11eAP and only few APs have currently enabled this feature.Relevant to backhaul networks, the fact that HCCA is notwidely deployed and that EDCA does not support guaranteedQoS. It necessitates careful design especially when the Wi-Finetwork is used to support residential broadband connectivityin addition to cellular backhaul traffic.B. WiMAX Network TechnologyWiMAX was originally designed for outdoor usage and isdefined in the IEEE 802.16 standards for broadband wirelesstechnologies. WiMAX is based on OFDM and provides higherthroughput and longer coverage range compared to Wi-Fi. Itcan operate in both unlicensed (typically 2.4 GHz and 5.8GHz) and licensed (typically 700 MHz, 2.3 GHz, 2.5 GHz,and 3.5 GHz) bands. Using unlicensed bands, it can reducethe CAPEX at the cost of interference problems. Comparedto microwave, WiMAX is less costly in terms of licensedspectrum fees (33). In addition, WiMAX can operate in nonLOS scenarios while microwave has strict LOS requirements.Backhaul applications can use the relatively simple 802.162004 standard for fixed connectivity applications, in point-topoint, point-to-multipoint, and mesh topologies.WiMAX supports relatively high data throughput. Theoretically, WiMAX can provide single channel data rates upto 75 Mbit/s and up to 350 Mbit/s via multiple channelaggregation (34). WiMAX can operate in various frequencybands which has effects on the bandwidth capacity and thedistance coverage. The higher the frequency, the greater thebandwidth capacity and the shorter the coverage range. MIMOand Adaptive Antenna Steering (AAS) can be used to enhanceWiMAX throughput. In addition, WiMAX supports bandwidthmanagement via centralized bandwidth scheduling in bothuplink and downlink directions. This allows efficient resourceallocation and hence higher achieved capacities (35). Usingorthogonal frequency division multiple access (OFDMA) concepts (a.k.a., sub-channelization), multiple subscriber stationscan communicate with the base station in the same timeslotover separate subchannels without interfering with each other(36). Recent WiMAX backhaul products such as in (37) canbe deployed as point-to-multipoint base stations with 6 sectorswith each sector supporting a throughput of 6 E1 links.WiMAX supports a maximum range of approximately 50km for single-hop architectures in the presence of line of sightand 25 km in non-line of sight connectivity scenarios (37).High-gain directional antennas can be used to significantlyincrease range and throughput. Packet overhead can be analyzed similar to Wi-Fi. The MAC Protocol Data Unit (PDU)overhead of WiMAX is 6 bytes (33; 38). Hence, when usingWiMAX to backhaul voice traffic, the backhaul voice overheadratio becomes 19% (6 / (25+6)) which is much more efficientthan Wi-Fi. This MAC overhead ratio can be reduced byconcatenating multiple Service Data Unit (SDU)s for the same

service into a single MAC PDU (33). On the other hand, thePHY header of WiMAX depends on various parameters suchas channel bandwidth, symbol time duration and coding rate.In (39), the PHY overhead of WiMAX was analyzed and itwas shown that WiMAX has a high PHY overhead ratio (morethan 50 %) (39). With the same concatenation techniques,multiple MAC PDUs can be put together into a single burstto save PHY bandwidth (33). Higher packing efficiency isindeed desirable for backhaul networks as it allows supportingmore E1/T1 connections per WiMAX link. However, for bothWi-Fi and WiMAX networks, concatenation should also beperformed such that it does not cause high jitter to delaysensitive traffic as well as to timing synchronization operation.Unlike standard Wi-Fi, WiMAX PMP mode can supportguaranteed QoS through centralized scheduling mechanismsfor different service classes (38). In WiMAX 802.16-2004standard, four QoS classes were defined including UnsolicitedGranted Service (UGS), Real Time Polling Service (rtPS),Non Real Time Polling Service (nrtPS) and best effort. AUGS connection communicates its traffic rate requirementsto the base station during connection establishment and thebase station allocates exactly such amount of bandwidth ineach frame whether it is used or not. This is similar to circuitswitched T1/E1 concept. rtPS uses dedicated periodic slots inthe uplink channel for sending bandwidth requests to the basestation; nrtPS connections use dedicated periodic request slots,but the allocation of dedicated requests is much longer thanrtPS connections. nrtPS connections may use contention-basedtime slots to send their requests to the base station. Thesecontention slots are also used by the best effort connections.IV. T IMING AND S YNCHRONIZATION A SPECTThis section discusses the fundamental challenge in wirelessbackhaul relevant to timing and synchronization. Currentcellular systems need frequency and time synchronization forproper operation. In our context, frequency synchronizationmeans that all local clocks at base station sites as well as radionetwork controllers are locked to a common highly accuratefrequency reference. On the other hand, time synchronizationrefers to the establishment of a common time reference amongnetwork nodes which ensures that base station clocks are inphase. For instance, requiring that two watches be frequencysynchronized means that they should tick at the same speedirrespective whether they show the same time (e.g., oneclock may show 8:00 and the other may show 9:00). Theclocks are said to be time synchronized only when they bothdisplay the same time (e.g., both show 8:00). Obviously, timesynchronization requires frequency synchronization. Table IIsummarizes frequency and time accuracy requirements in traditional 2G and 3G systems. More details on the requirements

Frequency Accuracy(parts per billion: ppb)50505050

TimingAccuracyN/AN/A2.5 s1 s GPS

for other cellular systems can be found in (40).

Frequency accuracy is always required in cellular systemsto discipline base station local clocks and hence maintainthe accuracy of the radio carrier frequency on the airlink.Misaligned radio carriers can result in undesirable interferencein the cellular network as well as violations of the allottedbandwidth (41). Frequency synchronization is also requiredthroughout the network in some systems such as GSM forsmooth handover in order to prevent call dropping and speechclipping problems. This is because during the handoff preparation phase, while a connection with the target base stationis being made, agreement with the target base station onnew frequency and timing offsets is required. The agreementprocess is much shorter (i.e., below 100 ms) when the current and target base stations are synchronized to the samefrequency reference and hence leading to reduced handoffduration (42). This aspect was confirmed in the experimentalstudy in (43), where it was shown that if two BTSs havea frequency offset greater than 100 part per billion (ppb),calls which successfully complete handovers entail a poorMean Opinion Score (MOS) value and high percentage ofspeech clipping. The results also indicated call dropping anddegraded quality as a result of the longer handoff durationdue to the long synchronization/agreement time with the targetbase station.On the other hand, timing accuracy is needed in cases wherebase stations require an accurate time reference, such as in theTime Division Duplex (TDD) mode for UMTS which timingalignment is critical to ensure minimal interference in thenetwork between base stations. Timing synchronization is alsoimportant to the operation of some systems like CDMA2000which derives all time critical CDMA transmissions andoperations such as pilot channels, pseudo noise sequences andWalsh functions from a highly accurate time reference e.g.GPS. Furthermore, some positioning services (e.g., time ofarrival, assisted GPS) depend on the presence of a commontime reference in all base stations to locate the user based onthe observed propagation delays from multiple base stations.Finally, timing synchronization is also needed in soft handoffscenarios in which a mobile can communicate simultaneouslywith multiple base stations. This guarantees that minimal softcombining buffers are required by the mobile device as allframes from all base stations in the mobiles active set arrive atsimilar times, and ensures that cross interference managementmechanisms between base stations operate properly (44).A. Timing and Synchronization in Circuit Switched NetworksIn TDM circuit switched networks, synchronization is animportant design objective. If the transmitter and the receiverare not synchronized, bit slipping problems may occur. For

This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.O. TIPMONGKOLSILP et al.: THE EVOLUTION OF CELLULAR BACKHAUL TECHNOLOGIES: CURRENT ISSUES AND FUTURE TRENDS

example, if the receivers clock is slower than the clock

on the transmitters side for 1 part per million (ppm), thetransmitter sent 1,000,000 bits but the output from the receiveris 999,999 bits (42). This problem exacerbates with increaseddata rates, such as in SDH/SONET systems. In these systems,to maintain timing synchronization, a hierarchical master/slaveclock system is usually implemented, where the master clock,with very high accuracy (1011 ), provides timing informationto slave clocks which they use to derive their timing. Thismeans that the master clock incurs one slip every 1011 tickswhen compared to the ideal clock.The ITU Recommendation G.811 suggests a four levelclock hierarchy (42; 45). Assume that we have three cascadeddigital switching exchanges with the master clock connectedto the first exchange and customer equipment with a slaveclock connected to the last. In this system, the clock of thefirst exchange is synchronized by the master clock while slaveclocks residing in the other exchanges6 synchronize their ratesto the timing of the received signal using digital phased lockedloops (DPLL). The process continues until the end device.Notice that the quality of the slave clocks accuracy degradesas recovered timing depends on the DPLL and its jitter bandwidth (which is very narrow). In our example, since the secondexchanges clock was not directly synchronized by the masterclock, it is referred to as stratum 2 clock. Similarly, sincethe clock in the third exchange is synchronized by a signalsynchronized to the stratum 2 clock in the second exchange,it is referred to as stratum 3 clock. The customer devicehas a stratum 4 clock. Thus, the clocks in the network aresaid to be traceable to the primary reference clock. Reference(46) provides a thorough survey of the historical evolution ofsynchronization systems including PDH, SDH/SONET, andATM.The main timing measures for traffic interfaces in TDMnetworks are jitter, wander and bit slip limits (47). ITU-TRec. G.823 and G.824 define the requirements for jitter andwander at synchronization interfaces based on the E1 and T1respectively (48; 49). In addition, ITU-T Rec. G.822 specifiesthe applicable slip rate objectives. When the clock of thetransmitting equipment is different from the clock recoveredat the receiving end, the slip buffer is needed (50).B. Timing and Synchronization in Packet Switched NetworksThe traditional packet switched networks do not needsynchronization to operate and hence provide no timing synchronization services. In order to allow timing distributionover emulated circuits, new techniques and protocols wereproposed to distribute reference timing signal. In packet networks, clocks can be recovered using adaptive and differentialmethods (47). Adaptive methods keep track of the inter-arrivaltime of timing packets or make observations to the arrivalbuffer levels to maintain time and frequency synchronization.In differential methods, we consider the fact that sometimespacket switched networks may have different primary clockreference from the carried TDM service clock. In this case,6 In many cases, a stand alone office clock is enslaved to the master clockreceived from the TDM network which in turn synchronizes the recoveredclock signals to the local exchanges clocks (46).

TABLE III: A Set of Four ITU-T Recommendations on

Description Defines the ways to deploy synchronizationover packet switched networks Sepcifies aspects of synchronization inpacket swithced (e.g. the maximum networklimits of jitter and wander) Outlines requirements for timing devicesused to support synchronization in Synchronous Ethernet, called Ethernet EquipmentClock (EEC) Addresses the specification of the packetbased clocks Specifies the requirements on Ethernettransport networks in timing aspects Details the timing architecture as well asdescribes how and where time and timing willflow through the architecture

Fig. 6: Synchronous Ethernet Concept

the difference between both clocks is encoded into the timing

packet stream at the sending packet network edge and recovered at the receiving edge. When the primary clock referenceis distributed and is made available to both ends (i.e., at thecore network and the base station sites such as in the case ofGPS), there is no need to recover timing.Table III summarizes the related ITU-T Recommendationson Timing and Synchronization. According to (47), there aretwo primary methods for reference timing signal distribution:Plesiochronous and network synchronous methods (e.g. Synchronous Ethernet) and packet-based methods (e.g. NTP, IEEE1588 PTP). We next describe the Synchronous Ethernet andthe packet-based methods in more details.1) Synchronous Ethernet is a technique recently proposedto distribute timing signal using a synchronous physical layer,in a similar fashion to TDM networks. In Synchronous Ethernet, each node recovers the clock information from theupstream node, filters jitter and wander via DPLL and thendistributes to downstream nodes via physical layer regardlessof the higher layer transmission protocols (see Figure 6).While conventional Ethernet relies on the reference frequencyprovided by the free-running operation of the local oscillatoron the order of 100 ppm (i.e., 104 ) (47), with synchronous

This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.10

IEEE COMMUNICATIONS SURVEYS & TUTORIALS, ACCEPTED FOR PUBLICATION

Fig. 8: IEEE 1588 and Synchronous Ethernet

Fig. 7: IEEE 1588 Exchange Messages

Ethernet, highly accurate and stable frequency is distributed at

the physical layer without impacting the Ethernet operation.The advantage of distributing a physical layer clock is that thetiming performance would not be affected by the impairmentsfrom the higher network layers, i.e., packet loss and packetdelay variations (51; 52). Synchronous Ethernet can satisfythe requirements of cellular networks timing accuracy asit was shown to achieve < 2 ppb accuracy in (51; 52).However, using Synchronous Ethernet poses a hard end-toend connectivity requirement between base stations and theradio controller to guarantee the traceability to a single masterclock. Consequently, network switches need to be upgradedto support Synchronous Ethernet. In addition, it is still achallenge when crossing multiple operator domains or havingto reach the base stations through non-synchronous networks.In such scenarios, packet based synchronization methods,(e.g., IEEE 1588 PTP), can be deployed to extend timinginformation to the end nodes (53).2) The packet-based methods rely on timing informationcarried by the packets (e.g. sending dedicated timestamp messages or two-way timing information transfer) (47). Timingpackets are not necessarily coupled with data packet streamsand are usually sent on a separate stream with their own QoSrules. The protocols used for packet network synchronizationare Network Time Protocol (NTP) and IEEE 1588 PrecisionTime Protocol (PTP). The NTP protocol (RFC 1305) usessoftware-based implementation for synchronization; that leadsto inaccurate estimation of timing information (54). In fact, thecurrently deployed NTP (RFC 4330: NTPv4) delivers timingaccuracy on the order of 10 ms which is not adequate forthe accuracy requirements in cellular networks. IEEE 1588PTP protocol addresses this shortcoming of NTP and providesmore precise timing information by utilizing hardware-basedschemes to accurately timestamp packets departure timesinstead of software-based methods which are susceptible todelay variations (54). PTP was shown to achieve frequencyaccuracy below the required 50 ppb as discussed in (51; 52).The IEEE 1588 PTP uses master/slave synchronizationparadigm (see Figure 7). The master clock sends a messageto its slaves to initiate the synchronization via multicast transmission; then each slave accomplishes the synchronizationby responding to the master. The synchronization process

can be divided into two phases. First, the master sends a

SYNC message to slave clocks; which receive the mastersSYNC message and note the packet arrival time (t2). Thenthe master sends a follow-up message containing a moreaccurate timestamp which carries the actual time at whichthe SYNC message departed the master clocks interface (t1).Now the slave clocks know master-slave difference Dms ,which corresponds to the clocks offset (O) plus master-slavepropagation delay, Pms . In the second phase, the slave clocksends a delay request message to the master (t3), then themaster timestamps the received message and sends it back tothe slave via a delay response message (t4). Now the slaveknows slave-master difference, Dsm , which corresponds tothe offset (O) plus slave-master propagation delay Psm . Ifsmthe ratio R denotes the ratio PPms , then it can be shown thatthe propagation delay from the master to the slave clocks is+Dsmgiven as Pms = Dms1+R. The clocks offset O is given asO = Dms Pms and the propagation delay from the slave tothe master Psm = RPms . When the link between the masterand slave clocks is symmetric, then R = 1, otherwise such asin the case of DSL links then the ratio R can be estimated byobserving packet inter-arrival time dispersion (see (55)).The "IEEE 1588 version 2" has more features than theoriginally released "version 1", including the addition of aunicast transmission feature and an increased message rate.This is because in the multicast model, the client has to listento all the delay-request and delay-response messages producedby or for other clients. This poses unnecessary load on theclient. To minimize the clients load, deploying multicast onlyfor SYNC messages and using unicast in the delay adjustphase may be more appropriate (56; 57). Furthermore, theincreased message rate reduces the impact of packet lossesand improves the accuracy resolution (56; 57).Packet based methods, such as IEEE 1588 can be combinedwith Synchronous Ethernet to circumvent its end-to-end connectivity requirement and allow traversing multiple domains.As shown in Figure 8, the RNC in the core network connectsto the base station sites via a Pseudowire connection over twocascaded domains: Synchronous Ethernet followed by MetroEthernet. Since the RNC synchronized output bit stream isfirst sent over the Synchronous Ethernet, the physical layerclocks are traceable to the same master clock. At the edgeof the Metro Ethernet network, timing is conveyed using PTP

This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.O. TIPMONGKOLSILP et al.: THE EVOLUTION OF CELLULAR BACKHAUL TECHNOLOGIES: CURRENT ISSUES AND FUTURE TRENDS

Fig. 9: TICTOC Network Model, adapted from (58)

protocol messaging. At the provider edge, TDM service clocks

are regenerated and conveyed to the base stations.Finally, we conclude this subsection by drawing the readersattention that there has been a recent effort in the IETF underthe Timing over IP Connections and Transmission of Clock(TICTOC) working group to propose a unified frameworkfor frequency and timing distribution over packet switchednetworks. This standard draft (58), points out the shortcomingsof the existing synchronization solutions and also outlines acomplete and robust time and frequency transfer system inpacket switched networks (58). The TICTOC architecture isbased on frequency distribution and time distribution layerscarried over the network by the timing protocol, e.g., NTP,IEEE 1588 PTP (see Figure 9). The slave clocks derive thefrequency accuracy from the master clock through a frequencydistribution protocol. Then, if the application requires time accuracy, the time acquisition module uses an accurate frequencyreference from the frequency layer and begins the master-slavetimestamp procedure (e.g., using IEEE 1588) (58).

11

receiving nodes have to synchronize their clocks according to

the timestamp in the beacon (61; 62). Through this process,the network-wide synchronization can be achieved. In (63),an implementation of IEEE 1588 PTP over Wi-Fi networkwas introduced in the ad hoc mode to provide more accuratetiming reference. The results, however, are only applicableto the accuracy level for instrumentation applications. Timingand synchronization for cellular applications needs furtherresearch while GPS based synchronization can be used in themeantime.WiMAX network needs synchronization in both TDD andFrequency Division Duplex (FDD) modes. For example, inWiMAX TDD frame structure, the Transmit/Receive Transition Gap (TTG) and Receive/Transmit Transition Gap (RTG)define the guard band time gaps. The base station needs tokeep clock synchronization accurate to prevent TDD framedrifts outside the guard bands and interferes with adjacentframes. The less accurate the clock source, the wider therequired guard bands (64). Timing and synchronization amongadjacent base stations is needed to minimize interference. TheIEEE 802.16 standard defines the use of GPS for timingsynchronization (38). In practice, the IEEE 1588 PTP maybe used as an alternative due to its lower cost. For highlyreliable synchronization in the network, IEEE 1588 PTP canserve as a backup timing reference in base stations deployedwith GPS receivers. If the GPS signal becomes unavailable,the base station can still perform synchronization using IEEE1588 PTP (64).V. C OMPARISON , T RENDS AND O PEN I SSUESIn this section, we start by summarizing and comparingall backhaul technologies discussed in the previous sections.We then address relevant future trends such as wireless meshbackhaul, and "All-IP" network designs including femtocells.A. Comparison Among Different Backhaul Technologies

C. Timing and Synchronization in Wi-Fi and WiMAX Networks

The IEEE 802.11 standard defines a simple protocol forclock synchronization (59) which depends on whether Wi-Finodes communicate in the infrastructure or ad hoc mode. Inthe infrastructure mode, Wi-Fi nodes communicate throughan access point (AP), whereas in the ad hoc mode, nodes cancommunicate directly, i.e, without an AP. When operating inthe infrastructure mode, a master/slave clock synchronizationmechanism is implemented where the AP acts as a master.The AP periodically transmits the beacon frame that includestimestamp information to which all of the slaves clocks haveto synchronize (60). On the other hand, when operating in thead hoc mode, synchronization is more complicated becauseall nodes must participate in the beacon generation process(61; 62). In this regard, each node chooses a random delaybefore it attempts to transmit a beacon. If it receives a beaconframe from another node before the delay expires, it doesnot transmit anything but simply adjusts its own clock to thetiming information from the received beacon frame. In the casethat the delay period finishes without hearing anything from itsneighbors, the node will transmit its beacon frame to which the

From Table IV, we can see that deploying leased T1/E1

copper provides suitable support for 2G GSM backhaul dueto its guaranteed QoS and timing reference. However, forthe UMTS 3G system which needs to support both voiceand high speed data traffic, leased T1/E1 copper becomesa cost-inefficient choice because the price increases linearlywith capacity. On the other hand, optical fiber transmissiontechnologies offer high capacity which is sufficient for 3Gand 4G cellular technologies. It can also guarantee QoS andprovide timing reference; however, this comes at cost of highinstallation and maintenance fees. Packet switched networks(e.g., Ethernet and IP) offer high capacity (depending on themedium) at significantly low cost. However, they incur lowpacket efficiency when carrying voice payloads, and do notnatively support QoS and timing synchronization.On the other hand, deploying wireless backhaul relaxesthe requirements of the availability of wired connectionsand their infrastructure installations. Microwave technologieswhich usually use synchronous transmission techniques (e.g.,SDH or nxT1/E1) can guarantee QoS, and provide timingsynchronization. It can offer data rates ranging from 2 - 170

Provides timing and synchronization

Has simple mechanism to

provide timing and synchronization but the accuracy level is insufficientfor backhaul applicationsThe standard defined using GPS as a referenceclock for network synchronization

High[75 Mbit/s(single channel)350 Mbit/s,multi-channel]

Guarantees QoS

Supports guaranteed QoS in PMP

mode

Mbit/s depending on the framing protocol and the modulation techniques. However, this radio link requires LOS forcommunications and poses high upfront CAPEX, in termsof costly equipment and licensed spectrum fees. Satellitebackhaul offers a solution for challenging areas where noother technologies can be implemented. It has a flexiblecoverage range and provides timing reference but suffers frompropagation delay and high costs compared to other methods.Wi-Fi and WiMAX are emerging as prospective wirelessbackhaul technologies due to their high throughput and longdistance coverage. However, Wi-Fi suffers from several disadvantages such as low packet efficiency, lack of QoS guarantees, and the lack of accurate synchronization mechanismswhich match the backhaul applications requirements. On theother hand, WiMAX has many features that outperform Wi-Fi.For example, it can guarantee QoS and provide more accuratenetwork synchronization, compared to rudimentary synchronization mechanisms in Wi-Fi. The 802.16-2004 standard usespositioning services (e.g., GPS) for network synchronizationand Wi-Fi backhaul may use GPS as a viable workaround.However, implementing IEEE 1588 PTP over WiMAX andWi-Fi for cost efficiency or as a backup timing reference inbase stations deployed with GPS receivers is still an activearea of research. Although WiMAX has a licensed spectrumfee, the standardization is expected to drive the total costdown. Nevertheless, Wi-Fi offers very low cost due to massproduction and unlicensed spectrum usage.

B. Wireless Mesh Backhaul

Multihop Wi-Fi and WiMAX, or wireless mesh networks,offer several advantages such as longer distance coverage andhigher throughput over long distances. For instance, trafficcan be routed around obstacles in non-LOS environments andhence effectively reduces the distance between transceivers atthe base stations and transceivers at the aggregation pointsusing mesh nodes. This can increase the network throughputdue to lower path loss and spatial reuse. In dense urban environments where microcells/ picocells are deployed, wirelessmesh backhaul can offer significant cost savings by connectingmultiple microcells and forwarding their traffic to a singlewired location (4).Wireless mesh backhaul, however, poses challenges in areas such as security, billing, resource reservation, QoS, andtiming and synchronization (6568). Relevant security challenges include network availability, confidentiality, integrity,and authenticity with goals to enable secure routing, intrusiondetection, and facilitate trust and key management (66). Billingmodels and accounting mechanisms are required when awireless mesh network is administered by third parties. In (67),the authors offer a bandwidth management and accountingmechanism that allows the base stations or radio networkcontrollers to dynamically claim and release bandwidth fromthe mesh network depending on the observed network load.The reservation for resources can be performed according tostandard mechanisms as in (69). Let us now briefly highlightaspects of multihop Wi-Fi and WiMAX technologies and

This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.O. TIPMONGKOLSILP et al.: THE EVOLUTION OF CELLULAR BACKHAUL TECHNOLOGIES: CURRENT ISSUES AND FUTURE TRENDS

discuss their QoS and synchronization challenges.

1) Wi-Fi mesh backhaul: Wi-Fi mesh backhaul can coverlonger distance, such as 80 km over a three hop Wi-Fi link (amax of 38 km for a single Wi-Fi link as demonstrated in (21)).In (70), the surveys of existing proposed methods to enhancethe channel utilization in Wi-Fi mesh are presented. Productvendors such as (4) have been offering proprietary Wi-Fi meshfor mobile operators backhaul. For example, (4) announceda wireless mesh backhaul solution for mobile operators whichsupports a capacity up to eight T1s or six E1s over asmany as five wireless hops. The IEEE standardization bodiesalso contributed to the development of Wi-Fi mesh networksby proposing the 802.11s standard (71). 802.11s addressesthe current shortcomings of IEEE 802.11 MAC protocol inmultihop environments, such as hidden and exposed terminalsand unfairness problems (72; 73). It also defines the Mesh Deterministic Access (MDA) as an optional MAC scheme whichintroduces a medium reservation for QoS (74). However, MDAbehaves similarly to EDCA for Wi-Fi single hops and cannotguarantee QoS. Achieving guaranteed QoS is a subject ofongoing research; for instance using RSVP-like techniquessuch as in (68; 75). On the other hand, synchronization inIEEE 802.11s standard is performed on a per-neighbor basisinstead of the more optimal global synchronization used instandard Wi-Fi networks as it may take a long time.2) WiMAX mesh backhaul: Similar to Wi-Fi mesh, WiMAXmesh can cover longer distance via multiple hops (76). TheWiMAX 802.16-2004 standard for fixed and nomadic accesscan be used for cellular backhaul applications since nodesare fixed. In addition, this standard supports both PMP andmesh topologies7. WiMAX mesh mode includes schedulingmechanisms for data transmission between the communicatingnodes. This can be achieved by using a centralized scheduling, distributed scheduling, or a hybrid of both schedulingschemes (77). Centralized scheduling algorithms are moresuitable for cellular backhaul applications because they canensure collision-free transmission, provide better QoS support,achieve better bandwidth utilization, and avoid hidden terminalissues (78; 79). Although the standards define schedulingmechanisms, they do not define service class concepts forthe mesh mode and leave details of the relationship betweenscheduling and the interference between mesh nodes open todesigners choice and research (8082). On the other hand,WiMAX mesh network synchronization requires a commontiming source which can be provided by either GPS or IEEE1588 PTP. An accurate clock reference allows the TDD modeto coordinate simultaneous transmissions on multiple linksthroughout the mesh. Similar to Wi-Fi mesh networks, timingand synchronization in WiMAX mesh is still open for furtherresearch.C. All-IP Network Trends and FemtocellsMobile operators and network providers are moving towardsall-IP networks in which all traffic leaving future basestations is natively based on IP protocols and is carried over7 Recently the 802.16m superseding the 802.16j was proposed as theframework for WiMAX mesh networks.

13

Fig. 10: IP-based Iub Interface Protocol Structure (84)

packet switched networks with no circuit emulation requirements (83). This concept would converge the core and accessnetworks which would result in cost savings, and simplermanagement and maintenance. In addition, each base stationcan be served by multiple radio controllers which increasessystem availability. A cross product of all-IP backhaul designis the introduction of femtocells which are home locatedcellular base stations at the size of a standard Wi-Fi accesspoint and provide connectivity to cellular users over their IPnetwork. For instance, a HSDPA femtocell can cover users attheir homes and backhaul traffic over an IP based broadbandconnection such as xDSL. Thus, femtocells can reduce theneed for adding traditional base station towers; leading toreduce backhaul establishment cost. Let us briefly describethe protocol aspects of IP based backhaul interfaces (based onthe Iub interface) and then shortly discuss aspects of femtocelltechnology.The backhaul Iub interface defines the interaction betweenNode Bs and RNCs (i.e., base stations and their controllersin UMTS terminology) over backhaul networks. The Iubinterface protocol structure for IP-based networks is definedin (84; 85) and is shown in Figure 10. There are two types oftraffic flows between RNC and Node Bs: signaling and userdata. Unlike GSM Abis interface based on E1/T1 time slots orATM deployments of Iub in UMTS networks, the IP based Iubinterface is natively based on IP protocols. Signaling is carriedover Stream Control Transmission Protocol (SCTP) while datais carried over UDP. UDP provides unreliable service andleaves the error checking and correction to the applicationlayer; while SCTP provides reliable service and in-sequencetransport of messages with congestion control features. Thisis in contrast to current AAL2 and AAL5 adaptation for dataand signaling traffic in ATM based backhaul UMTS networks.As such, the use of IP based backhaul simplifies managementand allows better incorporation of IP technologies for routing and QoS at the cost of more challenges in timing andsynchronization.Femtocells, also called home base stations, are low-costbase stations installed by the users for better indoor signalreception (86). These home base stations operate on lowtransmit power (100 mW or even 10 mW) and provide shortrange coverage (20 - 30 m) (87). They share the licensedspectrum allocated to cellular service providers (in the rangeof 1.9 - 2.6 GHz) and offer data rates from 7.2 - 14.4 Mbit/s(87). The femtocells communicate with the cellular network

This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.14

IEEE COMMUNICATIONS SURVEYS & TUTORIALS, ACCEPTED FOR PUBLICATION

to the projected large number of femtocells and the unicast

nature of PTP signaling, significant overload may occur at themaster clock in the core network. In addition, message securitymay result in high delay variations due to message protectionmechanisms which can impair the accuracy of time estimatedby IEEE 1588. Such issues are subject of further investigation.VI. C ONCLUSION

Fig. 11: A Simplified Femtocell Architecture

over a broadband connection such as xDSL. This technology

offers benefits to the users (e.g. better indoor signal reception)as well as the operators (e.g. lower operating costs).Figure 11 shows a simplified architecture for femtocells.Femtocells connect to the operators network over the Internetthrough a home gateway which includes security, firewall, andNAT functionalities. The operators network includes a security gateway which manages Virtual Private Network (VPN)tunnels (e.g., IPSec) with the femtocells and allows access tothe core network services including the PTP clock server (88).The clock server acts as a PTP master clock reference whilethe femtocells act as slaves. The synchronization process isbased on packet-based methods as described earlier. However,deploying femtocells raises technical challenges such as interference, QoS, security, and synchronization.Since the femtocells operate on the same frequency bandas macrocells, femtocell devices may face adjacent channelinterference with the macrocell. A less significant interferenceissue arises also due to interference among femtocells themselves when placed in close proximities (87). However, suchinterference can be controlled due to the low-power designof femtocells and by proper femtocell placement. In addition,when using home backhaul connection, femtocells share thebackhaul bandwidth with other IP services over public IPnetworks. This can dramatically affect the throughput andQoS of femtocell services. It also poses security issues tothe transfer of cellular channels and more importantly to thetransfer of timing information. The security of the timinginformation is an issue as manipulating time or frequencyinformation can cause service disruption. In addition, it isgenerally easier to pose denial of service attacks on femtocellsthan macro base stations and hence disrupts their operation.The physical security of the femtocell itself is also importantas users can tamper with the femtocell hardware to createman-in-the-middle attacks which allow channel hijacking.Finally, synchronization is a major challenge due to xDSLasymmetric bandwidth and the load on the core PTP timeservers. As femtocells are likely to use xDSL, the conventional IEEE 1588 PTP may not work properly due to thesymmetrical links assumption. The study in (55) shows theperformance degradation of the IEEE 1588 PTP when workingin asymmetrical links and proposes a mechanism to estimatethe asymmetry ratio among the forward and reverse directionsas we discussed in Section IV-B. According to (88), due

This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.O. TIPMONGKOLSILP et al.: THE EVOLUTION OF CELLULAR BACKHAUL TECHNOLOGIES: CURRENT ISSUES AND FUTURE TRENDS

This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.O. TIPMONGKOLSILP et al.: THE EVOLUTION OF CELLULAR BACKHAUL TECHNOLOGIES: CURRENT ISSUES AND FUTURE TRENDS

Said Zaghloul is a PhD candidate at the Technische

Universitaet Carolo-Wilhelmina zu Braunschweig,in Brunswick, Germany. Prior to his PhD studies,he was with Sprint-Nextel as a TelecommunicationDesign Engineer where he made contributions to thedesign and testing of Sprints wireless data networkarchitectures in several areas including MVNO androaming solutions, dual GPRS/CDMA solutions andhybrid WiFi/CDMA phone products. In 2003, Saidwas granted a Fulbright Scholarship to pursue hisMSc studies at the University of Kansas. In 2005,he received his MSc degree with honors in computer engineering, for his workin the area of inversely multiplexed satellite connections. In 2002, he receivedthe first IEE award for BSc senior projects in Jordan for his work dedicatedto the development of a UMTS cellular planning tool. Saids current researchinterests include: next generation all-IP wireless architectures, signaling planeperformance, mobility, and wireless communications.

Admela Jukan is W3 Professor of Electrical and

Computer Engineering at the at the Technische Universitaet Carolo-Wilhelmina zu Braunschweig, inBrunswick, Germany. Prior to coming to Brunswick,she was research faculty at the Institut Nationalde la Recherche Scientifique (INRS), University ofIllinois at Urbana Champaign (UIUC) and GeorgiaTech (GaTech). From 2002-2004, she served as Program Director in Computer and Networks SystemResearch at the National Science Foundation (NSF)in Arlington, VA. She received the M.Sc. degreein Information Technologies and Computer Science from the Polytechnic ofMilan, Italy, and the Ph.D. degree (cum laude) in Electrical and ComputerEngineering from the Vienna University of Technology (TU Wien) in Austria.Dr. Jukan is the author of numerous papers in the field of networking, andshe has authored and edited several books. She serves as a member of theExternal Advisory Board of the EU Network of Excellence BONE. Dr. Jukanhas chaired and co-chaired several international conferences, including IFIPONDM, IEEE ICC and IEEE GLOBECOM.